"Well, the ones that stay in trees grab on with their feet and their thumbs at the same time," she explains patiently, "so it's a hard landing. The ones that roost in caves just use their back feet, so they don't hit the rock as hard."

Meanwhile, Peter's tiny enough that it's hard for her to judge where he might roost. He could fit anywhere. "What sounds nicer to you - a cave or a tree?" Maybe his bat-self has a preference.